The present invention relates to a system for transmitting the power of an engine to four wheels of a four wheel drive motor vehicle having a central differential, and more particularly to the system which provides multiple operational ranges of the transmission.
In a conventional four-wheel drive motor vehicle having a central differential comprising bevel gears or a planetary gear device, the central differential is disposed in alignment with an output shaft of an automatic transmission. The automatic transmission is provided with a hydraulic control system and various sensors for detecting driving conditions of the motor vehicle. A fluid operated multiple-disk friction clutch is provided for controlling the differential operation of the central differential. A torque of the clutch is variably controlled in accordance with hydraulic pressure from the hydraulic control system or output signals from a microcomputer for controlling a torque split ratio between the front wheels and the real wheels of the vehicle, thereby improving the power of the engine, driveability, stability at braking, and antilock brake operation.
Japanese Patent Application Laid-Open 63-176728 discloses a four-wheel drive motor vehicle in which a central differential comprising a simple planetary gear device is provided. An output of a transmission is transmitted to a carrier of the simple planetary gear device. The torque is split to front wheels through either of a sun gear or a ring gear and to rear wheels through the other. The torque to the front and rear wheels is unequally split at the ratio determined by the difference between pitch circles of the sun gear and the ring gear. A fluid operated multiple-disk friction clutch as a locking device is provided for controlling the differential operation.
Japanese Patent Publication 53-32466 discloses a power transmitting system for a motor vehicle. The system is provided with two sets of simple pinion planetary gear devices, a pair of complex planetary gear devices, a pair of clutch devices, and four restricting devices for providing four forward speeds including an overdrive and one reverse speed of the transmission.
However, in the conventional systems, it is necessary to provide a sub-transmitting system for providing a further operational range having a smaller gear ratio. Consequently, the transmission becomes large in size.